1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting a plurality of multiplexed video programs over a channel of allowing for a constant bit rate (CBR), and particularly to an encoding apparatus and its method for multiple video programs transmission in which the sum of the bit rates of all the multiplexed video programs is constant, but each of the video programs allows for a variable bit rate (VBR) by synchronously controlling each of video encoders by means of a central controller.
2. Description of the Prior Art
Generally, when a plurality of video programs are transmitted through a single channel of allowing for only the CBR, each of them is encoded independently by it's own video encoder at the CBR compression, and then is multiplexed to transmit the aggregate bit stream.
Such a prior art will now be explained in detail with reference to FIG. 1. FIG. 1 shows a blockdiagram of a video encoding apparatus for transmitting multiple video programs. The video encoding apparatus includes an encoding unit 110 for encoding video programs received from each of video sources 1 to video sources N according to bit rate control by means of respective bit rate controllers 121, 123, and 125, a bit rate controlling unit 120 for controlling bit rates of the respective encoders 111, 113, and 115 according to buffer informations from the respective buffer 131, 133, and 135, a buffer unit 130 for storing outputs of the respective encoders 111, 113, and 115 and transferring them to a multiplexer (MUX) 140, the MUX 140 for multiplexing the outputs of the respective buffers 131, 133, and 135, and a network interface 150 for transmitting the output of the MUX 140 through a channel of allowing for a CBR transmission.
In the prior art apparatus shown in FIG. 1, every encoder 111, 113, or 115 includes independently buffers 131, 133, and 135 and bit rate controller 121, 123, and 125 to execute independent coding for compression.
There are, however, two main shortcomings with independent coding at a constant bit rate. First, the program content can vary significantly from program to program as well as within a program. This variation can lead to large fluctuations in picture quality among programs and within a program, so that a picture of an acceptable picture quality can not be provided with all users fairly. Second, if bandwidth is allocated in order to guarantee the acceptable picture quality with respect to all video programs, unnecessarily wide bandwidth may be allocated to video program having simple picture. That is, when the same rate is used for all types of pictures regardless content, bandwidth or channel capacity is effectively wasted, so that the number of programs capable of transmitting over a single channel is reduced.